The present invention relates to a turbine helper drive apparatus which couples a turbine to an AC motor and adds a generation torque of the AC motor to that of the turbine in a positive or negative direction to drive a load.
A large-scale compressor is driven by a steam turbine, for example. If the capacity of the compressor is to be increased, the capacity of the turbine becomes short, and a large-capacity turbine must be employed to increase the capacity. Alternatively, if the ambient temperature is high during, e.g., the summer months, the efficiency of the turbine is decreased and hence, its output capacity is decreased.
Due to recent discounts in nighttime electric power costs, a motor drive is more economical than a steam turbine during nighttime operation.
For this reason, a demand has arisen for coupling a motor to the turbine to be used as a supplementary power source for the turbine. In order to meet this demand, the present inventors proposed a turbine helper drive apparatus wherein an AC motor, advantageous in high-speed operation, is coupled to a turbine, and an output of the AC motor is controlled to be a predetermined value through a power converter, thereby sharing part of a load of the turbine by the motor.
This proposal corresponds to the following patent applications:
(i) U.S. Patent Application Ser. No. 943,766 filed on Dec. 19, 1986;
(ii) EPC Patent Application No. 86117516.4 filed on Dec. 16, 1986; and
(iii) Canadian Patent Application No. 526,073 filed on Dec. 22, 1986.
The above three patent applications have the same contents. All the disclosures of these patents are incorporated in the present application.
Generally, the turbine must output at least its minimum power for the sake of self-cooling, and of course, cannot generate a negative torque.
For this reason, a drive system using a turbine has the following problems. Due to the characteristics of the turbine described above, a minimum load is required, and hence, a nonload operation of the system cannot be performed. In addition, when the system is stopped or decelerated, a deceleration rate is determined depending on a load state, and hence, abrupt deceleration and abrupt stop cannot be performed.
In a conventional helper drive apparatus, when field-weakening control of the accompanying AC motor is performed, precision of power control is undesirably degraded.